Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($) Integer	Jun. 30, 2021 USD ($) Integer	Jun. 30, 2022 USD ($) Integer	Jun. 30, 2021 USD ($) Integer	Dec. 31, 2021 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,391,600,000		$ 1,391,600,000		$ 1,203,000,000
Unfunded Commitments	700,000		700,000
Accrued Investment Income	705,600,000		705,600,000		602,700,000
Loan Foreclosure Expense	0	$ 0	0	$ 0
Commitments to Fund Commercial Mortgage Loans	10,000,000		10,000,000
Real Estate Held-for-sale	$ 40,000,000		$ 40,000,000		40,900,000
Repurchase agreements - Typical Days outstanding			30
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%		102.00%
Gross Securities Loaned	$ 297,300,000		$ 297,300,000		283,700,000
Gross Securities Borrowed	112,000,000.0		112,000,000.0		94,800,000
Outstanding Repurchase Agreements	0		0		0
Equity Securities
Schedule of Investments
Unrealized Gain (Loss) on Investments	(1,400,000)	1,500,000	1,000,000	4,200,000
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments	26,900,000	$ 32,000,000	43,200,000	$ 59,000,000
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	196,300,000		196,300,000		198,600,000
Cash Collateral for Borrowed Securities	112,000,000		112,000,000		94,800,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,499,100,000		2,499,100,000		2,568,700,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	8,400,000		8,400,000		8,300,000
Accrued Investment Income	$ 7,800,000		$ 7,800,000		8,100,000
Financing Receivable, Modifications, Number of Contracts | Integer	0	0	0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 0		$ 0		0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0	$ 0	0	$ 0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments					100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,125,100,000		1,125,100,000		987,900,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	5,100,000		5,100,000		9,300,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,120,000,000		1,120,000,000		$ 978,600,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	$ 47,000,000		$ 47,000,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	796		796
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	50		50
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	122		122
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	6		6